The IL-2 receptor and related cytokine receptor systems are being studied to clarify the T cell immune response in normal, neoplastic, and immunodeficient states. Following T-cell activation by antigen, the magnitude and duration of the T-cell immune response is determined by the amount of IL-2 produced, levels of receptors expressed, and time course of each event. The IL-2 receptor contains three chains, IL-2Ra, IL-2Rb, and gc. Dr. Leonard cloned IL-2Ra in 1984, we discovered IL-2Rb in 1986, and reported in 1993 that mutation of the gc chain results in X-linked severe combined immunodeficiency (XSCID, which has a T-B+NK- phenotype) in humans. We reported in 1995 that mutations of the gc-associated kinase, JAK3, result in an autosomal recessive form of SCID indistinguishable from XSCID and in 1998 that T-B+NK+ SCID results from mutations in the IL7R gene. Based on work in our lab and others, gc was previously shown to be shared by the receptors for IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21. We also previously characterized genes that were induced or repressed by IL-2, IL-4, IL-7, and IL-15. T helper cell differentiation is critical for normal immune responses, with Th1 differentiation being important for host defense to viruses and other intracelllular pathogens, Th2 differentiation being vital in allergic disorders and related to helminths, and Th17 differentiation being vital in a range of inflammatory disorders, including psoriasis and inflammatory bowel disease. We previously showed that IL-2 is important for Th2 differentiation and reported that IL-2 regulates expression of the IL-4 receptor in a STAT5-dependent manner and critically controls priming of cells for Th2 differentiation. Moreover, using genome-wide Illumina-based ChIP-Seq (chromatin immunoprecipitation coupled to DNA sequencing) analysis, we had discovered broad regulation of Th2 differentiation via STAT5A and STAT5B. Moreover, we had discovered that IL-2-mediated IL-4Ra induction was critical in priming cells for Th2 differentiation. We then had substantially extended these findings by showing that IL-2 via STAT5 induces expression of IL-12Rb1 and IL-12Rb2 and that the induction of IL-12Rb2 is critical for Th1 differentiation and we defined the mechanism of regulation of IL-12Rb2. Additionally, we previously showed that IL-2 via STAT5 also regulates the T box protein, T-bet, and that in contrast to the induction of IL-12R proteins, IL-2 inhibits expression of IL-6Ra and gp130, helping to explain the inhibition of Th17 differentiation. Consistent with the ability of Tbx21 to inhibit Th17 differentiation, expression of Tbx21 in Th17 cells resulted in increased IFNg but decreased expression of IL-17A. These results indicated a very broad effect of IL-2 via STAT5 on T helper cell differentiation. In the current year, we had a major paper reporting the critical role of IL-2 in Th9 differentiation. We demonstrated a direct effect of IL-2 on Th9 differentiation via its induction of STAT5 binding to the Il9 promoter. Moreover, we showed opposing actions of IL-2 and IL-21 in Th9 differentiation based on their differential regulation of BCL6, which is induced by IL-21 but repressed by IL-2. We also have continued our studies of the role of STAT5 tetramerization in vivo. We previously collaborated with Dr. K. Christopher Garcia at Stanford on a project in which the three dimensional structure of IL-2 complexed to its receptor was compared to that of IL-15 bound to its receptor. These studies had been published in Nature Immunology and provided key mechanistic and structural insights into the functional differences between IL-2 and IL-15. During the current year, we continued our collaboration with Dr. Garcia, studying the actions of wild type IL-2 versus novel IL-2 variants, a project with potential clinical ramifications. Some of the IL-2 variants are potent inhibitors of both IL-2 and IL-15, and some have partial agonistic activity. These may have clinical potential, including in cancer and in immunological diseases. Moreover, the approach used to generate these novel IL-2 variants should be broadly applicable to other gc family cytokines and potentially other type 1 cytokines as well. We also have the long-term goal of identifying new causes of inherited human immunodeficiency. Overall, these studies help to improve our understanding of signaling by gc family cytokines. These findings clarify basic molecular mechanisms that are relevant to normal and pathological immune cell function, including allergy, autoimmunity, and cancer.